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Quantum computers could break the cryptography that conventional blockchains rely on. Now physicists say a way of entangling the present with the past could foil this type of attack.

A blockchain is a mathematical structure that stores data securely over time. The idea has risen to fame on the back of the Bitcoin boom. Bitcoin relies on blockchains to securely store its related currency transactions.

But the same technology can store any kind of data—shipping data, the progress of computer programs, smart contracts, and so on. Indeed, blockchains look set to become one of the enabling technologies of the 21st century.

And yet they have an Achilles’ heel. The security of a blockchain is guaranteed by standard cryptographic functions. These are relatively secure because breaking them requires huge computing resources, which are not generally available.

That looks set to change with the emergence of powerful quantum computers. It will be child’s play for such devices to break this kind cryptographic protection. But quantum computers cannot break quantum cryptographic codes, so various groups have suggested adding quantum cryptography to blockchains to guarantee their security.

There is a better, more fundamental solution, say Del Rajan and Matt Visser at the Victoria University of Wellington in New Zealand. Quantum cryptography merely adds a quantum layer to the standard blockchain protocol. Instead, they suggest making the entire blockchain a quantum phenomenon.

Their idea is to create a blockchain using quantum particles that are entangled in time. That would allow a single quantum particle to encode the history of all its predecessors in a way that cannot be hacked without destroying it. Such a protocol relies on the laws of physics to guarantee security. However, it also leads to somebody unusual side effects. “This decentralized quantum blockchain can be viewed as a quantum networked time machine,” say Rajan and Visser.

First some background. A blockchain is simply a ledger that records information of a certain type—currency transactions, for instance. The transactions are continually added to a database called a block, but at the end of a given time period, the block is encrypted using a mathematical device called a hashing function. This produces a unique number that can be used to represent the data exactly.

This unique number is then included in the next block with the next set of transactions. After a time, it is all encrypted using the hashing function to produce a new unique number. This is added to the next block. And so on, creating a chain of blocks that are all nested inside the latest one—hence the name blockchain.

Anybody attempting to falsify the historical record would need to find a way to alter the data in a way that does not change the outcome of the hashing function. And that is so computationally challenging that it is considered impossible with a classical computer. But it is possible with the kind of quantum computers that will soon be available.

So Rajan and Visser have come up with a different approach that relies on a fully quantum version of a blockchain. The phenomenon at the heart of their approach is called entanglement. When two quantum particles are entangled, they share the same existence. This happens when they interact at the same point in space and time. After that, a measurement on one immediately influences the other, no matter how far apart they may be.

What guarantees security is that entanglement is extraordinarily fragile. A measurement on one of a pair of entangled particles immediately destroys the link. So if a malicious user attempts to interfere with one of the pair, it is immediately obvious to the other.

Just as particles can become entangled across space, they can also become entangled over time. So a particle existing in the present can be entangled with one that existed in the past. And a measurement on it immediately influences its predecessor.

That leads to some subtle and counterintuitive phenomena. For example, there is a special quantum sense in which it becomes possible to influence the past. Of course, there are strict limits on what this makes possible. It’s not possible, for example, to set in train a series of events that will kill your grandparents, thus ensuring you never existed. That kind of paradox isn’t allowed.

But it does become harder to distinguish between cause and effect. Another effect is that it becomes possible to increase the amount of information that can be transmitted through time.

It is this type of temporal entanglement that Rajan and Visser exploit to produce a quantum blockchain. The basic idea is to encode data on a quantum particle. This becomes the first quantum block.

When more data is available, this is combined with the data from the first particle in a quantum operation that entangles it with a second particle. The former is then discarded, and the record of the first block of transactions is combined with the second block. The data from a third block can be added in the same way, creating a chain.

This chain is secure because anybody attempting to tamper with it immediately invalidates it. That’s the advantage of quantum entanglement.

This quantum blockchain has another advantage: the earlier blocks are completely tamper-proof. “The attacker cannot even attempt to access the previous photons since they no longer exist,” say Rajan and Visser. “Entanglement in time provides a far greater security benefit than an entanglement in space.”

What’s more, most of the technology to make this work already exists, at least in proof-of-principle form. “All the subsystems of this design have already been shown to be experimentally realized,” say Rajan and Visser.

That’s interesting work that is likely to become more relevant as powerful quantum computers begin to emerge. IBM already has a 50-qubit quantum computer, and more powerful machines are in the pipeline. It’s only a matter of time before they become capable of undermining trust in blockchains.

But a key part of the infrastructure necessary to make this kind of quantum blockchain work is not yet available: a quantum web. This is a network that can transmit quantum information via quantum routers without destroying its quantum properties. This kind of system is currently being designed and expected to be rolled out in Europe, the US, and China in the coming months or years.

Indeed, the job of building such a system is essentially an engineering task rather than one of fundamental physics. So it’s just a matter of time before a quantum blockchain becomes possible. Whether it will be this protocol that emerges as the best is another question, of course.

Perhaps Rajan and Visser could put their quantum time machine to good use by finding out what technology eventually triumphs in the future!

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